1. Field of the Invention
The present invention relates to a slip control system for a vehicle and, more particularly, to a slip control system for the vehicle, adapted to prevent a slip of the driven wheels on a road surface from becoming excessive.
2. Description of Related Art
Prevention of a slip of the driven wheels from becoming excessive at the time of accelerating or the like is effective in order to efficiently provide a propulsive force of the vehicle as well as to provide safety in terms of prevention of a spin of the vehicle body and so on. In order to prevent an excessive amount of a slip of the driven wheels, torque to be applied to the driven wheels as a cause of slipping may be reduced.
Slip control of this type is disclosed, for example, in Japanese Patent Unexamined Publication Nos. 170,740/1987 and 22,948/1982 and U.S. Pat. Nos. 4,484,280 and 4,583,611 (corresponding to Japanese Patent Unexamined Publication (kokai) Nos. 16,948/1983 and 56,662/1985, respectively).
In performing slip control, the torque to be applied is reduced by adjusting a load of the engine with smoothness in control, mileage or the like taken into consideration. For an engine of an Otto type, it is preferred to reduce a throttle valve opening angle and, for a diesel engine, it is preferred to reduce an amount of fuel injection. In this case, as a load adjusting means, such as a throttle valve, is operated by the accelerator, the load adjusting means is forcibly driven upon slip control in the direction of reducing the torque to be generated by the engine, prior to operation of the accelerator.
It is to be noted, however, that, if the driven wheel slips to an extremely great extent, the situation may occur wherein the opening angle of the load adjusting means, such as the throttle valve, is caused to become nearly zero and the throttle valve might be fixed in the position close to zero for some reasons. In order to avoid this situation, a control-unreactive area is provided so as not to cause an operational amount of the load adjusting means to reach nearly zero, i.e., to cause the opening angle of the throttle valve to come to nearly zero, prior to slip control, with the attempt to ensure the least possible running when the accelerator is nearly full open. More specifically, if the load adjusting means is the throttle valve, it is considered to set a control-unreactive area such that the throttle valve opening angle is caused not to be reduced to less than 25%, for example, by means of the slip control when the accelerator is full open.
It should be noted, however, that there is the occasion that the throttle valve opening angle be required to become smaller than 25%, i.e., 10% or smaller, for example, from the viewpoint of the requirement for slip control. In this case, it can be noted that the torque to be applied to the driven wheels becomes greater than the value required by the slip control by 15% of the throttle valve opening angle to be cancelled by the control-unreactive area. As a result, convergence of a slip of the driven wheels may be worsened.